1. Field of the Invention
This invention relates to a process and a system for increasing para-xylene recovery and production from a hydrocarbon feedstream comprising C8 aromatics. In particular, the process and the system comprise xylene isomerization and pressure swing adsorption to form a desorption effluent comprising a para-xylene enriched product. Para-xylene is then recovered from this desorption effluent.
2. Description of the Related Art
Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are often present together in a typical industrial C8 aromatic product stream from a chemical plant or a refinery. For instance, commercially available Mobil Selective Toluene Disproportionation and Mobil Toluene Disproportionation processes may produce such a stream. Naphtha reforming plants also produce these aromatics. Commercial examples include POWERFORMING and PLATFORMING processes. It is also possible to convert C3/C4 hydrocarbons into aromatics via a CYCLAR process. These C8 aromatics are also produced in large quantities in oil refineries, which produce gasoline, diesel fuel, heating oil, and other fuels. Benzene and toluene, having lower molecular weights than the C8 stream, are two other large volume valuable aromatic products produced from some of these chemical plants and refineries. (PLATFORMING and CYCLAR are registered trademarks of UOP, Inc.)
Among the four C8 aromatic compounds related to the present invention, all having the same molecular formula C8H10, EB is used primarily for making styrene by direct dehydrogenation, oxidative dehydrogenation, or conversion via an ethylbenzene hydroperoxide intermediate, a co-product from an “Oxirane” process for producing propylene oxide. Styrene is a large volume monomer for producing many important polymers such as polystyrene and styrene-butadiene rubbers. However, largely for economic, logistic, production control and product purity reasons, most EB feedstocks used in typical styrene production plants are produced on purpose by alkylation of benzene with ethylene, not by recovery from a C8 aromatics stream from a chemical plant or an oil refinery. It is not unusual that the total amount of EB from a typical C8 aromatic stream is not significant enough to justify installing additional facilities for its recovery and purification as a byproduct. Accordingly, it is often desirable, sometimes necessary, to remove, convert or otherwise dispose of EB in a most economic manner.
Of the three xylene isomers, PX has the largest commercial market. PX is used primarily for manufacturing purified terephthalic acid (PTA) and terephthalate esters such as dimethyl terephthalate (DMT), which are used for making various polymers such as poly(ethylene terephthalate), or PET, poly(propylene terephthalate), or PPT, and poly(butene terephthalate), or PBT. Different grades of PET are used for many different popular consumer goods such as films, synthetic fibers, and plastic bottles for soft drinks. PPT and PBT may be used for making similar products with different properties.
While OX and MX are also useful as solvents or raw materials for making products like phthalic anhydride and isophthalic acid respectively, demands for OX and MX and their downstream derivatives in the market place are much smaller and more limited. Because of the much higher demand for PX as a feedstock than the demands for OX and MX, it is usually more desirable commercially to increase or even maximize PX production from a particular source of C8 aromatic materials. Otherwise, there could be substantial overproduction of MX and/or OX and inadequate production of PX, thus creating an imbalance of supplies and demands in the various C8 aromatics markets.
There are two major technical challenges in achieving this goal of increasing or maximizing PX yield and/or production from a particular process or plant. First, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights. Second, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is limited, at most, to how much PX is originally present in the C8 aromatic stream unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. Therefore, increasing the PX yield and improving the PX production efficiency by using different and novel technologies or processes are two objectives constantly sought after by the chemical and refining industries and the technology community.
Fractional distillation is a commonly used method for many processes in many industrial plants to separate chemicals. However, it is often difficult to use such a conventional fractional distillation technology to separate the EB and different xylene isomers efficiently and economically. This is because the boiling points of the four C8 aromatics fall within a very narrow 8° C. range, from about 136° C. to about 144° C. (see Table I). The boiling points of PX and EB are about 2° C. apart. The boiling points of PX and MX are only about 1° C. apart. As a result, large equipment, significant energy consumption, and/or substantial recycles would be required to provide effective and satisfactory xylene separations.
TABLE IC8 compoundBoiling Point (° C.)Freezing Point (° C.)ethylbenzene (EB)136−95para-xylene (PX)138  13meta-xylene (MX)139−48ortho-xylene (OX)144−25
Notwithstanding, various methods and processes, other than simple fractional distillation, to separate these C8 aromatic components into individual products have been tested and developed, and some are successfully practiced in commercial scales. Examples include fractional crystallization, adsorption, and combinations thereof.
Fractional crystallization in a crystallizer takes advantage of the differences between the freezing points and solubilities of the C8 aromatic components at different temperatures. Due to its relatively higher freezing point, PX is usually separated as a solid in such a process while the other components are recovered in a PX-depleted filtrate. High PX purity, a key property needed for satisfactory conversion of PX to PTA and/or DMT commercially in most plants, can be obtained by this type of fractional crystallization. U.S. Pat. No. 4,120,911 provides a description of this method. A crystallizer that may operate in this manner is described in U.S. Pat. No. 3,662,013. Commercially available processes and crystallizers include crystallization isofining process, continuous countercurrent crystallization process, direct CO2 crystallizer, and scraped drum crystallizers. Due to high utility usage and the formation of a eutectic between PX and MX, it is usually more advantageous to use a feed with as high an initial PX concentration as possible when using fractional crystallization to recover PX.
A different xylene separation method uses molecular sieves, such as zeolites, to selectively adsorb para-xylene from the C8 aromatic feedstream to form a PX-depleted effluent. The adsorbed PX is then desorbed by various ways such as heating, stripping, and others. (See generally U.S. Pat. Nos. 3,706,812, 3,732,325 and 4,886,929) Two commercially available processes used in many chemical plants or refineries are PAREX and ELUXYL processes. Both processes use molecular sieves to adsorb PX. In such molecular-sieve based adsorption processes, a higher amount of PX, typically over 90%, compared with that from a fractional crystallization process, typically below 65%, may be recovered from the PX present in a particular feed. (PAREX is a registered trademark of UOP Inc.; ELUXYL is a registered trademark of Institut Francais du Petrole).
Depending on the effectiveness of a particular separation method or system, these PX depleted streams or filtrates may still contain various amounts of residual PX. At the same time, MX, OX and EB concentrations are higher than those in the original C8 aromatic feedstocks. The actual EB concentration may vary substantially, depending primarily on (a) the separation method, (b) the feedstock composition and (c) the isomerization catalyst and the isomerization conditions in the isomerization reactor when the PX-depleted streams are passed or recycled through one or more xylene isomerization steps.
For many of these PX separation processes, the higher the original PX concentration in the feedstream is, the easier, more efficient and more economical it becomes to perform the PX separation. Therefore, there are strong economic and technical incentives to increase the PX concentration in a hydrocarbon feedstream comprising the C8 aromatic compounds prior to sending the feedstream to a PX separator such as a PAREX unit or a fractional crystallizer discussed above.
As discussed in the preceding paragraphs, PX may be separated by different methods such as fractional crystallization or selective adsorption. Without additional processing steps, however, the total amount of recoverable PX is still limited. This is because EB and the three xylenes are usually present in concentrations close to those dictated by the thermodynamic conditions under which they are produced, due to their inter-convertibility under such production conditions. It is not unusual that the PX concentration is not more than about 25 mol % (equivalent to 25 wt %), and MX, at about 50 mol %, of the total C8 aromatics present in a typical aromatic product stream produced in a refinery or a chemical plant. Thus, many industrial aromatic processes provide additional steps to recycle and to isomerize the various PX depleted streams coming from the separation step to produce more PX by isomerizing OX, MX and sometimes EB to PX.
Due to its chemical properties, EB may be destroyed partially or completely under certain xylene isomerization conditions. As discussed later, when EB is destroyed during xylene isomerization, it is usually converted into benzene and ethane in the presence of and with consumption of hydrogen. Due to their very different physical and chemical properties, benzene and ethane can be easily separated from the xylenes by many conventional methods. With or without EB destruction, an isomerization product effluent from a xylene isomerization reaction becomes a part of the feedstream to the PX separator, such as a crystallizer or an adsorption unit.
Regardless the specific systems of and the catalysts selected for these isomerization processes, the PX concentrations in the isomerization effluents from the isomerization reactors are dictated primarily by thermodynamics, i.e. within the equilibrium concentration limits of PX under the isomerization conditions. Similar to the situation discussed earlier involving C8 aromatics streams directly coming from refineries or chemical plants, it is desirable to increase the PX concentrations in the xylene isomerization effluents to levels higher than those dictated by xylene isomerization thermodynamics (super-equilibrium concentration), prior to sending the isomerization effluents, as the feeds, to PX separation units. As before, this higher PX concentration would allow better utilization and/or de-bottlenecking of the existing unit and equipment, such as a fractional crystallizer, for PX separation.